Digitizers, or digitizing tablets as they are sometimes called, are used for generating digital signals representative of the positions of coils relative to active areas defined by grids of conductors. The coils are carried, for instance, in the tips of hand-held or machine-held, movable instruments (e.g., pointers, pens, styluses, or cursors).
A typical digitizer conductor grid has a first group of parallel, spaced conductors oriented generally in an X-direction, and a second group of parallel, spaced conductors oriented generally in a Y-direction perpendicularly to the first group, thereby defining an orthogonal X-Y coordinate system. The conductors can be formed, for instance, by wires or by paths of conductive ink disposed on a non-conductive substrate.
The digitizer also typically has an alternating-current (AC) source, such as an oscillator, and signal detection circuitry. The AC source supplies an AC signal of a predetermined frequency and amplitude, for instance, to the coil of the movable instrument. The coil signal is inductively coupled to the conductors of the grid, thereby inducing AC signals in the conductors. The detection circuitry senses the induced AC signals, and generates a signal indicative of, for example, its magnitude and, in some digitizers, other signal characteristics as well. A signal processor in the detection circuitry processes these signals to determine the position of the coil with respect to the grid.
The conductors are connected to the detection circuitry by multiplexers (MUX's). The MUX's and the detection circuitry typically are provided on a single printed circuit board, which can be called the "digitizer PC board."
Significant characteristics of digitizers include the size of the grid active areas and the exhibited resolutions in determinations of coil positions. The grid active-area size generally depends on the number and arrangement of conductors used in the digitizer grid and the inter-conductor spacing within the grid. The digitizer resolution generally depends on the conductor arrangement, the inter-conductor spacing and the type of detection circuitry employed.
It has been found that the grid conductors can be arranged advantageously in various "loopback" patterns within the grid. In a loopback pattern, conductors in the digitizer grid are looped repeatedly back and forth across an active area of the grid. Loopback patterns increase the active area of the grid for a specified digitizer resolution, or increase the digitizer resolution for a specified grid active area. In either case, loopback patterns generally minimize the number of grid conductors and the number of MUX's required, and, thus, realize savings in component costs.
Digitizers incorporating various, alternative loopback patterns have been proposed in the prior art. For instance, U.S. Pat. No. 4,661,656 to Rodgers discloses a digitizer in which three main conductors A, B, and C in each of the X and Y conductor groups are disposed in the following loopback pattern (as one moves from one end of the grid to the other):
A+, B+, C+, A-, B-, C-, A+, B+, C+, etc. PA0 0, 3, 2, 1, 4.
where the signs designate directional polarities in each loop of the conductors. This scheme has the effect of dividing the grid active area into a plurality of equal-width sections; each section contains, e.g., six segments of the conductors, the first three being of positive (+) polarity and the next three being of negative (-) polarity.
Rodgers uses auxiliary conductors connected to the just-described looped conductors through a series of resistor taps to identify the particular section of the tablet that contains the coil. Rodgers then finds the looped conductor with the lowest absolute induced signal value, and the looped conductor with both the next lowest absolute induced signal value and a polarity reversal to determine the position of the coil within the given section. Unfortunately, the auxiliary conductors represent complexities in the design and implementation of the digitizer, and, therefore, increase the costs of the device.
Commonly assigned U.S. Pat. No. 4,734,546 to Landmeier discloses a digitizer system employing a conductor grid with a loopback pattern, in which the grid is divided into a plurality of sections. The conductors are passed through the sections in a predetermined order so that the combination of the directional polarities of adjacent conductor segments is different in each section. Due to these known and differing directional polarities, each section of the grid active area is uniquely identifiable and distinquishable from the others, even though formed by the same conductors.
Consequently, this arrangement reduces the number of separate conductors required to span the grid active area and the number of multiplexers required to couple the conductors to the detection circuitry. For example, by looping the conductors in pairs through four equally sized quarter sections of the grid, as taught in the illustrative embodiment of that patent, 16 conductors in each group can be used to span the same active area that would require 64 conductors without a loopback arrangement. Moreover, one 16-input multiplexer can be used for each group that would require four such multiplexers absent a loopback arrangement, all without the need for a group of auxiliary conductors.
Commonly assigned U.S. Pat. No. 4,831,216 to Landmeier discloses an improvement over the earlier Landmeier patent, in which the digitizer system employs primary and secondary interleaved sets of conductors. The primary set of conductors is formed by looping spaced conductors in pairs through different sections of the grid active area in such a way that the directional polarities of each pair are different in each section. The secondary set of conductors is formed by looping at least one additional conductor back and forth through the active area in the spaces between the conductor pairs of the primary set.
In a digitizer employing a loopback arrangement of that type, adjacent conductor segments are spaced from one another typically by about 0.4 inch (one centimeter) or less. Thus, it is possible to span an active area extending about 25.2 inches (64 centimeters) (i.e., 63 spaces times 0.4 inch) using 16 conductors and one 16-input multiplexer.
For larger grid active areas, either the spacing between adjacent conductor segments is increased, or additional conductors and additional multiplexers is used. In the former case, accuracy is sacrificed, while in the latter case, costs associated with components and manufacturing increase.
U.S. Pat. No. 4,835,347 to Watson discloses the use of a digitizer grid, which accommodates large grid active areas without sacrificing accuracy or significantly increasing costs. The conductors in each group in the Watson patent are separated into first and second sets of conductors, each such set including a different number of conductors. For example, 15 conductors can be used in each direction, separated into a set of seven conductors and a set of eight conductors.
In the loopback pattern in accordance with that patent, the individual conductors in the first and second sets shift in position relative to one another. For example, at the extreme left of the active area, conductor A of the second set is to the immediate right of the conductor 1 of the first set. In their next crossing, conductor A of the second set is to the immediate left of conductor 1 of the first set. This positional shifting, which is due solely to the fact that each set includes a different number of conductors, continues all the way to the extreme right of the active area.
In the example mentioned above, where each conductor group has an 8-conductor set and a 7-conductor set that are looped and interleaved across the grid active area as just described, and where adjacent conductor segments are equally spaced from one another by 0.4 inch, unique positional and directional polarity relationships between individual segments are obtained for up to 14 crossings of the first set. This permits the active grid area to be as large as 89.6 inches (228 centimeters) (14 first set crossings times 0.4 inch). Thus, the Watson approach provides more than three times the grid active area achievable with the scheme of the Landmeier '216 patent.
It would be desirable to provide a digitizer employing a loopback pattern realizing most or all of the advantages of Watson, while providing an even more flexible design of the grid active area that can be readily manufactured and provide accurate and reliable measurements of coil positions.